1. Field of the Invention
This invention relates to the field of testing formations surrounding an earth borehole with a formation testing tool to obtain fluid samples and, more particularly, to improvements in downhole determination of formation fluid properties.
2. Background of the Invention
Existing well logging devices can provide useful information about hydraulic properties of formations, such as pressures and fluid flow rates, and can obtain formation fluid samples, generally for uphole analysis. Reference can be made, for example, to U.S. Pat. Nos. 3,934,468 and 4,860,581. In a logging device of this general type, a setting arm or setting pistons can be used to controllably urge the body of the logging device against a side of the borehole at a selected depth. The side of the device that is urged against the borehole wall typically includes a packer which surrounds a probe. As the setting arm extends, the probe is inserted into the formation, and the packer then sets the probe in position and forms a seal around the probe, whereupon formation pressure can be measured and fluids can be withdrawn from the formation.
Wireline formation testing, in general, strives to provide, inter alia, a measurement of the formation mobility. Formation mobility is defined as the formation permeability, measured in darcy, divided by fluid viscosity, measured in centipoise. The value of fluid viscosity, under in situ conditions of pressure and temperature, is usually unknown. However, to be able to accurately identify the local formation permeability, knowledge of viscosity is required. Knowledge of fluid density measurement would also be very helpful, for example in identifying fluid type or types.
Downhole determination (i.e., performed, in whole or in part, while the tool is downhole) of properties of sampled fluids, including density and viscosity, has been proposed in the prior art. This would permit greater flexibility in measuring and/or determining fluid properties, at essentially in situ conditions and, on fluid samples from various depth levels. However, equipment and techniques contemplated for achieving same, suffer one or more of the following disadvantages: undue complexity, inaccuracy or unreliability of measurement, and/or incompatibility or difficulty of adaptation for use with existing formation testing tools.
It is among the objects of the present invention to provide an improved method and apparatus for downhole determination of properties of sampled borehole fluids.